Field of the Invention
The present invention relates to a driving mechanism including a sensor that measures force acting on a link joined to the driving mechanism, a robot apparatus measurement method and a robot apparatus control method, and a component manufacturing method.
Description of the Related Art
In recent years, multijoint robots have been used on various industrial product production lines. However, there are many processes that are difficult to perform with multijoint robots. For example, on production lines for assembling, e.g., automobile components, multijoint robots are widely used particularly in processes in which a load of several hundred grams to several kilograms is imposed on a component. On the other hand, there are various difficulties in performing processes requiring a load provided to a component to be around several grams such as attachment of a work including, e.g., a soft object, a lightweight object or a low-strength member and processes for providing a precise fit with multijoint robots.
As stated above, in processes that cannot be performed with multijoint robots, currently, dedicated apparatuses or tools specialized for those processes are used instead of multijoint robots. However, such dedicated apparatuses or tools are designed and manufactured only for a particular process or a work to be handled, and thus there is a problem in that significant time and costs are spent until an apparatus or a tool of such type is actually prepared and, for example, a production line is started.
Therefore, there is a demand for performing a process in which a fragile work including, e.g., a soft object, a lightweight object or a low-strength member such as mentioned above is handled, using a versatile multijoint robot rather than a dedicated device and/or a dedicated tool.
Where a work including a soft object, a lightweight object or a low-strength member such as mentioned above is handed, for example, for prevention of breakage or deformation of the work, it is impossible to cause a large force to act on the work. Therefore, if a work of this type is manipulated with a multijoint robot, it is necessary to control force acting on the work with high precision via a joint or a link.
For example, a configuration in which a force sensor is disposed together with an end-effector such as a hand or a gripper attached to a distal end of a multijoint robot has conventionally been known. An output value from this force sensor is fed-back for driving control of the end-effector, whereby force acting on a work can be controlled. Also, in addition to the end-effector at the end, for example, it is conceivable that forces acting on respective links included in an arm of the multijoint robot are measured and fed back for driving control of the multijoint robot. In particular, a force that is necessary to be measured for high-precision driving control of a multijoint robot from among forces acting on links of a robot arm is a torque acting around a driving axis.
As a measure for detecting a torque acting on a link of an arm such as mentioned above, a configuration in which a torque sensor is mounted on a joint of a robot arm is proposed (for example, Japanese Patent Application Laid-Open No. 2011-72186).
Gravitational force, inertial force and/or Coriolis force, which act on a link itself, and/or force from an adjacent link act on a joint of a multijoint robot arm depending on a motion of the arm. For example, the forces acting on the joint include respective components of a total of forces in six directions including translational forces in three coordinate axis directions and rotative forces around three coordinate axes in an orthogonal coordinate system where a driving axis of the joint is a z-axis. In the below, from among the forces in the six directions, forces acting in five directions other than a force acting around the driving axis of the joint is referred to as a force in another axial direction.
On the other hand, in driving control of a multijoint robot arm, for example, a force around a driving axis of a joint, the force acting on a link, is detected and fed back for driving of the joint. Thus, it is desirable that a force sensor mounted on a joint can correctly detect a force around a driving axis of the joint, the force acting on a link joined to the joint.
However, upon a force in another axial direction such as mentioned above acting on a force sensor, the force sensor fails to correctly detect a force around a driving axis. For example, if a force in another axial direction acts on a force sensor of a type that includes a deformable part and determines the force by detecting an amount of deformation occurred in the deformable part, the force sensor deforms also in a circumferential direction of the driving axis of the joint from the effect of the force in the other axial direction.
Hereinafter, the effect of the force in the other axial direction on the force sensor is referred to as “interference in another axial direction”. In other words, upon some kind of deformation occurring in the force sensor as a result of a force in another axial direction being exerted on the force sensor, the deformation appears as a detection error of the force sensor that detects a force around the driving axis of the joint. In other words, a measurement error caused in a force sensor provided on a driving axis of a certain joint by a force in another axial direction as stated above is referred to as “interference in another axial direction”. If such interference in another axial direction occurs, it is impossible to correctly detect a force around the driving axis of the joint with the force sensor.
Therefore, in order to correctly detect a force around a driving axis of a joint, for example, it is necessary to correct a sensor detected value error caused by interference in another axial direction in some way. Therefore, for example, it is conceivable that a force in another axial direction acting on a force sensor is detected to correct a detected value from the force sensor.
However, a conventional joint structure such as described in Japanese Patent Application Laid-Open No. 2011-72186, a bearing is disposed between a force sensor and a link, and thus, it is not so easy to detect a value of a force in another axial direction.
A reason of difficulty in detection of a value of a force in another axial direction where a mechanical element such as a bearing is interposed at a joint of a multijoint robot arm like the conventional configuration in Japanese Patent Application Laid-Open No. 2011-72186 is provided below.
For example, this type of joint structure allows motion of a joint in a desired one direction only, and uses a bearing such as a cross roller bearing as a constraining unit (constraining part) for constraining motion in another direction. Such structure may result in complexity of a transfer pathway of a force in another axial direction.
For example, depending on the joint structure, there may be a pathway on which a joint driving force is transferred other than a joint axis connecting two links. For example, in the structure indicated in Japanese Patent Application Laid-Open No. 2011-72186, a force in another axial direction acting on a drive-side link is transferred via both a bearing, which is a constraining unit of the joint, and the force sensor. With such configuration, it is difficult to obtain a correct value of the force in another axial direction transferred to the force sensor.
In particular, a driving force of a link reduced by the amount of a frictional force of a bearing, which is a constraining part of a joint, is transferred to a force sensor. Thus, in order to correctly grasp a force in another axial direction acting on the force sensor, it is necessary to grasp the frictional force of the bearing disposed on the joint. However, a frictional force of a bearing of a joint exhibits non-linear characteristics relative to various factors such as a force acting on the bearing, a driving speed of the joint and an individual specificity of the bearing, and thus it is difficult to correctly grape a frictional force of a bearing.